


Risk

by papipachie



Category: Undertale (Video Game)
Genre: Ball Gags, Biting, Blood, Dom and Sub undertones, Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Fantasizing, Gaster sans and papyrus are brothers, I tried to make reader gender neutral, Light Masochism, Light Sadism, Magic, Other, Rough Sex, Sex, Swearing, author is just really gross please pardon them, gaster is one tol boi, just a little, risky sex
Language: English
Status: Completed
Published: 2018-02-11
Updated: 2018-02-11
Packaged: 2019-03-17 01:18:05
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,214
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/13648401
Author URL: https://archiveofourown.org/users/papipachie/pseuds/papipachie
Summary: Risky sex had always been a dirty little shadow in your heart. Luckily, you have a boyfriend who knows just how to exploit it.





	Risk

**Author's Note:**

> Hey whoa what hi hell yes am I involved in more than one fandom. Hell yes do I write gross porn for more than one fandom. I've got a thing for tall science boys so me writing Gaster porn was a given. I wrote six pages of smut on my phone, so pardon any mistakes! As usual, have fun!

Ever hate how much you love something?

 

Gaster pressed you up against the kitchen counter, kissing you deeply. Your arms clung to his neck while your legs wrapped around his lower body. The skeleton used his various hands to squeeze and grope at you, massaging your tender, blushing flesh. You two tossed around the dominant and submissive roles quite often, but you had played top for a while now, so it surprised you when he attacked you as soon as you came home from work, already half-naked and raring to take your clothes off as well. Well, maybe it wasn’t _that_ much of a surprise.

    

You let out a low groan as a bone-cold hand slid between your slick, hot thighs. The naughty hand lightly touched and teased your dripping heat for a few moments, then travelled up to your chest, ghosting over your dusky pink nipples. Gaster nipped at your ear, whispering softly into it.

    

“I love you. So much. Me being able to make you so happy under my touch...I feel so excited.” his voice, deep and rich, sank into your senses. His words made you blush, and Gaster smiled into your soft flesh, murmuring sweet nothings. You knew he wouldn’t stay like this, though. Soon, the beast would emerge, bringing almost destructive kinky pleasure. He would make sure that you would never be satisfied by another human being ever again. Only him. Gaster would make you scoff at a human’s advances, already knowing that he could please you far better than any man or woman could. Sure, a human might know your body’s functions and workings better, but Gaster knew _your_ body. What made it tick. What buttons to push, what levers to pull.

 

His hands ran up and down your back as you kissed him a little harder, licking at his teeth, fangs not yet unsheathed. The skeleton purred in return, low and rumbling in his cavernous chest.

 

A knock at the door.

 

You abruptly jerked away, startling the skeleton. Your pulse raced and goosebumps trilled across your skin.

 

“yo kiddo, you home?” a few minutes of terse silence. You wouldn’t really be able to respond anyway, seeing as a thick purple tongue was currently down your throat. Gaster _really_ liked the idea of risky sex. Being caught at any moment only made the act that more lustful and exciting. His magic buzzed with arousal and he lightly sucked on your tongue. You were barely able to contain a shaky gasp.

 

“guess not.” as soon as Gaster thought his brother was out of earshot, he continued.

 

“My, my.” he whispered against your flesh, gently scraping his now unsheathed teeth against your neck. “Think of Sans’ reaction to us...to me, undoing you. How _dirty,_ isn’t it? My oldest _brother_ watching us fuck?”

 

The skeleton rarely ever swore so carelessly, and it made shivers tingle down your spine.

 

“Po-Posi-sitively dis-ghh...” you moaned out, not even able to finish your sentence.

 

“What do you think he would do? Turn around and leave without fanfare? Snap a photo? Or perhaps…” his nails gently ran across your sides, goosebumps breaking out onto your skin. “Would he stay? Watch in awe? Jerk off, maybe, to our passionately rough love?”

 

“I, I d-don’t know…” you said breathily. “Sh-Should we i-invite hi-im?”

 

“No...there’s no _fun_ in that. Besides, you’re all mine.” Gaster’s voice dropped to a growl. To punctuate his words, he bit into your shoulder. You gave a small yelp, pulse fluttering. “Don’t ever forget that.” then he licked at the wound with surprising gentleness. “Not this time, at least. I have a feeling Sans would’ve found our last session much more... _interesting._ ”

 

You smirked at that, then your voice fell into a soft coo. “Mm, I think so, babyboy. I would’ve put on a real show for him. I would’ve shown Sans how much you really are the littlest sibling.”

 

You felt his grip tighten as he took in a wavering breath. “Don’t get smart with me now.”

 

“I wouldn’t dream of-nngh…” your words disappeared from your brain as he bit on your clavicle, deeper, and scraping his teeth right on your bone, the thin skin breaking easily under his long teeth. He pulled back, watching the blood pool in the small dip on the base of your neck and run down the center of your chest.

 

Looking straight at you, Gaster dipped low and took his sweet time languidly licking up your blood, almost inaudibly groaning against your body, hot breath fanning across your skin.

 

“Mmmh...you taste _so_ _good_...” he moaned out, reaching your pecs. His wide purple tongue licked at your painfully hard nipples. “So sweet,” he sucked on the pert muscle, lightly twisting the other one. “So warm and exotic. You still have that virgin taste...creamy and tender…”

 

You rubbed your legs together, his words making arousal whine needily at your mind as you keened in a high-pitched voice. Gaster gently yet firmly bit at your nipple, and you squealed at the overstimulation on the area, muscles clenching tightly. The skeleton sucked on it like a baby to a teat, the irony taste of your blood dancing across his tongue again. Even just this was bringing you closer to orgasm.

 

Gaster knew it, and he let up on his foreplay. He wanted you to last more than two seconds after, of course. He restrained himself to loving kisses peppered on your mouth, cheeks, forehead, neck and shoulders. He sucked beautiful purple bruises into your skin like ink blooming on paper as you nuzzled his forehead with a smile.

 

You two continue the impromptu makeout. As sweet and almost horrendously sappy as it is, you have to mutter into his mouth, “Move your arm,” you scoot back on the counter, the unusually warm, hard bone pressing on your thigh. You tilt your head a little. “Why there, isn’t it uncomfortable? Besides, why is it so hot-”

 

“Because that’s not my arm.” Gaster purred, his words candy-sweet and lustful, a Cheshire cat’s grin on his face

 

_Oh._

 

A deep blush swept up your neck and cheeks.

 

Gaster seemed to get a devilishly naughty idea, because his tongue flicks across his fangs (such a small move, yet somehow incredibly arousing…) and his hands lift you up and off the counter. He presses you to the cold kitchen wall, which is diagonal to the door and only about three feet away. You instinctively wrap your legs around his waist, one hand gripping the edge of the wall and the other his shoulder for support.

 

“Don’t worry, I got you.” he reassures sweetly, and summons a few hands that stroke your hair for extra emphasis. “But yes, you still need to hold on tight.” another grin, _oh wait when did he become naked_ and the royal purple behemoth was hot on your slick thighs, wetness already on the tip, dribbling down to your core as he pushed your knees upwards and inwards a bit, getting as close to you as possible.

 

You gasped quietly, never really getting over the capabilities of magic. You couldn’t really think about it either, Gaster’s monster gently rubbing over your hole was driving you mad. The skeleton saw your eyes flick to the door, mere feet away, and he smiled, a wicked light in his eyelights.

 

“U-Ughhh...ju-ust fuck m-me, _please._ ” you hissed out, squirming closer to him.

 

He smirks. “I think you can beg better than that.”

 

“Fuck me before I cut your dick off and use it as a dildo.” you said bluntly, not having any of that ‘begging’ shit.

 

Gaster blinked in surprise before regaining his composure. “I’m going to ignore that magical error. Rude humans need punishment, don’t they? Treat me with respect before I resort to washing your mouth out with soap. Wait a minute...” his eye flared with magic and a translucent orb of magic firmly worked its way into your mouth. “That’s better.” you wiggled your hips involuntarily, _that_ was the kinky G you really wanted to see.

 

~~(hey that rhymed)~~

 

“You’re lucky I love you,” he says, gently pushing the bulbous head of his swollen cock in you with a small groan. “Or I would’ve held out a lot longer.”

 

 _Haha, like you would’ve been able to._ You think smugly to yourself, grinning around the gag.

 

Your walls clenched and fluttered around his cock as he slowly sheathes himself inside you, taking care to make sure you weren’t in too much pain.

 

Gaster takes a handful of your ass to leverage himself against you, and starts thrusting, moaning along with you.

 

His cock was hot and throbbed inside you with every thrust, and you cried out from behind the gag, saliva dripping down the corners of your mouth. You were only able to make animal-like noises, but that was fine by you. You didn’t need to talk to show your affection.

 

Gaster steps it up a notch, angling his hips to hit your sweet spot, and when he found it, he went all out, pistoning inside you like a...well, like a monster.

 

You shrieked at the ecstasy-riddled assault on your body, and Gaster hissed out a few words in guttural Wingdings.

 

 _S-So -so go-ood! I lo-love y-you so m-uuhhch…!_ You were able to translate, but the rest had rambled off into nonsense.

 

“Child? Are you home?”

 

The two of you froze.

 

The lilting voice of Toriel came through the door again. “Sans had come a little over an hour ago and you aren’t answering your phone. Are you alright?”

 

Gaster smirked widely, and wiggled his hips inside you at an agonizingly slow pace. He rolled his pelvis under you, flicking your nipples. You convulsed, jaw clenched tightly around the magic to keep a noise from bubbling up and out of your throat as you breathed calmly and deeply through your nose.

 

The dirty little shit begins to thrust again, and you hear Toriel’s feet shuffle as he does so, icy cold fear spiking in your veins. The sharp contrast to your hazy, hot arousal is, well...even _more_ arousing, goosebumps once again jumping on your skin. You two could so _easily_ get caught...and by Toriel no less. Gaster had his face buried into the crook of your neck, biting at it and near silently gasping in pleasure.

 

You let scenarios run wild in your mind as you heard the goat woman walk off.

 

What if she tested the doorknob? You distinctly not remember getting a chance to lock it. She could step right in and see you, pressed against the wall, flushed and shimmering with sweat, beautiful purple marking your skin and deep crimson crusted around bite marks on your creamy skin, and the gag in between your supple lips. She would see Gaster, the feral look in the scientist’s eyes, his dastardly grin, full of shiny white fangs, his huge cock between your legs, spread wide to accommodate it as it stuffed you to the limit…

 

Or what if she brought the others to see the situation? Would Gaster, spurred by his ability to remain quiet, start fucking you fast and hard? Would you not be able to contain yourself, musical howls breaking free from your mouth, for them to hear? Would they be enraptured in your symphony of sin?

 

_You dirty, dirty fucker._

 

Maybe they’d burst in at the right moment, at the peak of the heat, at the _crescendo_ of your lovemaking! Witness the very culmination of your beings emerge as you two orgasmed, in sweet, sweet oblivious bliss while the others, embarrassed that they saw such an intimate moment, flew down the hall of your apartment building.

 

Imagine the hot shame you’d feel at being discovered...seeing Toriel and Alphys, and most likely all of your friends. Ooh. _Papyrus._ The sweet, naive, childlike monster witnessing your filthy, yet incredibly loving act...simply _evil._

 

Perhaps Gaster, emboldened at this discovery, would own up to it, causing a string of heated, maybe even a little _violent_ sessions of dominating you.

 

How delicious these all were.

 

“Wh-what are you thinking about?” Gaster panted, beginning a bruising pace that made you keen. He gave a strained smile as you bit down on the gag. “La-ater, then?”

 

It was clear you two were reaching the end of this. The knot in your belly twisted and clenched as it threatened to be unraveled. Gaster’s pace grew erratic and his moans more frequent. His cock pulsed and rippled. He growled in effort and slammed into your sweet spot, fangs latching onto your slightly bloody neck, moaning and gasping as he came, hot magic shooting into you, cum spilling down your thighs. You spasmed, vision going blurry for a moment as you cried his name, (best you could) for probably the best orgasm in your life.

 

You two dropped onto the cool kitchen tile, and Gaster cupped your face in his large hands, looking slightly woozy but blissed out. He stroked your cheek with his thumb, murmuring out a faint, ‘I love you’. The gag dissolved.

 

“Love you too baby.” you croaked out, leaning forward to give him a short, sweet kiss. “But I’m sweaty as hell. I have to shower.”

 

Gaster smiled crookedly. “May I join you?”

 

“What do you think?”  
  


**Author's Note:**

> Thanks for reading! Constructive criticism is always welcome in this gross household.
> 
> My weird Tumblr with occasional shitposts and story updates: https://papipachie.tumblr.com/


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